Magnetic field measurement apparatus, magnetic field measurement system and magnetic field measurement method

ABSTRACT

A magnetic field measurement apparatus includes a first gas cell disposed in a +z direction when seen from an object to be measured, a second gas cell disposed in the +z direction when seen from the first gas cell, a first measurement unit which measures a component of a magnetic field in the first gas cell, a second measurement unit which measures a component of a magnetic field in the second gas cell, a magnetic field generation unit which generates the magnetic field toward the second gas cell so as to reduce the component measured by the second measurement unit, and an output unit which outputs a signal in response to the difference in the components respectively measured by the first measurement unit and second measurement unit.

BACKGROUND

1. Technical Field

The present invention relates to the technical field of a magnetic fieldmeasurement apparatus, a magnetic field measurement system and amagnetic field measurement method.

2. Related Art

A magnetic sensor using optical pumping is used for a magnetic fieldmeasurement apparatus or the like which measures a magnetic field whichis generated from a living body. The magnetic sensor includes a cellwhich is filled with an atom such as alkali metal in a gaseous state.When pump light which has a circular polarization component isirradiated to the cell, the filled atom is excited. Subsequently, whenprobe light which has a linear polarization component is irradiated soas to cross the pump light, the excited atom rotates the polarizationsurface of the linearly polarized light which is included in the probelight in response to a magnetic field which is applied from the outside.The magnetic sensor measures the magnetic field by measuring arotational angle of the polarization surface of the probe light whichhas passed through the cell.

However, in a case of measuring a particular weak magnetic field such asa magnetic field, which is generated from the inside of a living body,by the magnetic field measurement apparatus, there is a problem in thatthe magnetic field from the outside of the living body may affect themeasurement.

As techniques which cancel the external magnetic field, techniques whichinfer and cancel a disturbing magnetic field on the basis of the amountof the magnetic field which is measured using a magnetic sensor aredisclosed in JP-A-2008-282983, JP-A-2003-332781 and JP-A-2000-329836. Inaddition, in JP-A-2007-170880 and JP-A-2001-51035, a gradiometricmagnetic sensor, which cancels the influence of the magnetic fieldswhich are not from the target to be measured and would become noise, bymeasuring gradient polarization components thereof using a plurality ofcoils, is disclosed.

SUMMARY

An advantage of some aspects of the invention is to measure a magneticfield of an object to be measured without influence from disturbancewith respect to a measurement unit which irradiates light to a gas celland measures a component in a predetermined direction of the magneticfield in the gas cell thereof even when there is disturbance whichexceeds a range which is measurable by the measurement unit.

According to an aspect of the invention, there is provided a magneticfield measurement apparatus including: a first gas cell which isdisposed in a predetermined direction when seen from a position in whichan object to be measured is installed; a second gas cell which isdisposed in the direction when seen from the first gas cell; a firstmeasurement unit which irradiates light to the first gas cell andmeasures a component in the direction of a magnetic field in the firstgas cell; a second measurement unit which irradiates light to the secondgas cell and measures a component in the direction of a magnetic fieldin the second gas cell; a magnetic field generation section whichincludes the object to be measured, the first gas cell and the secondgas cell interposed along the direction and generates a magnetic fieldtoward the second gas cell so as to reduce the component measured by thesecond measurement unit; and an output unit which outputs a signal inresponse to the difference in the components which are respectivelymeasured by the first measurement unit and the second measurement unit.According to the configuration, it is possible to measure the magneticfield of the object to be measured without influence from disturbancethereof with respect to the measurement unit which measures thecomponent in the predetermined direction of the magnetic field in thegas cell thereof by irradiating the light to the gas cell even whenthere is disturbance which exceeds a range which is measurable by themeasurement unit.

Moreover, according to another aspect of the invention, there isprovided a magnetic field measurement system including a plurality ofthe magnetic field measurement apparatuses in each independentdirection. According to the configuration, it is possible to measure thecomponents in the plurality of the directions in the magnetic field ofthe object to be measured without the influence from the disturbanceeven when there is the disturbance described above.

In addition, it is preferable to provide the magnetic field measurementsystem in which the plurality of the directions is orthogonal to oneanother. According to the configuration, even when there is thedisturbance, it is possible to measure each component in a rectangularcoordinate system in the magnetic field of the object to be measuredwithout the influence of the disturbance.

Furthermore, according to a further aspect of the invention, there isprovided a magnetic field measurement method of irradiating light to afirst gas cell which is disposed in a predetermined direction when seenfrom a position in which an object to be measured is installed andmeasuring a component in the direction of a magnetic field in the firstgas cell by a first measurement unit; irradiating light to a second gascell which is disposed in the direction when seen from the first gascell and measuring a component in the direction of a magnetic field inthe second gas cell by a second measurement unit; generating themagnetic field toward the second gas cell so as to reduce the componentwhich is measured by the second measurement unit by a magnetic fieldgeneration section which includes the object to be measured, the firstgas cell and the second gas cell interposed along the direction; andoutputting a signal in response to the difference in the componentswhich are respectively measured by the first measurement unit and thesecond measurement unit. According to the configuration, it is possibleto measure the magnetic field of the object to be measured withoutinfluence from disturbance thereof with respect to the measurement unitwhich measures the component in the predetermined direction of themagnetic field in the gas cell thereof by irradiating the light to thegas cell even when there is disturbance which exceeds a range which ismeasurable by the measurement unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a block diagram which illustrates an outline of a magneticfield measurement apparatus according to an embodiment of the invention.

FIG. 2 is a diagram which illustrates disposition of configurationelements of the magnetic field measurement apparatus according to theembodiment of the invention.

FIG. 3 is a diagram which illustrates disposition of configurationelements of a magnetic field measurement system in which two magneticfield measurement apparatuses are provided.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a block diagram illustrating an outline of a magnetic fieldmeasurement apparatus 90 according to an embodiment of the presentinvention. FIG. 2 shows a diagram illustrating disposition ofconfiguration elements of the magnetic field measurement apparatus 90according to the embodiment of the invention. In order to describedisposition of each configuration of the magnetic field measurementapparatus 90, space in which the magnetic field measurement apparatus 90is disposed represents as xyz right hand's coordinate space. In thiscoordinate space, a positive x direction indicates a direction in whichan x component increases, a negative x direction indicates a directionin which the x component decreases. In the same manner, a positive ydirection, a negative y direction, a positive z direction and a negativez direction are also defined with respect to y and z components.

A frame which is shown by a dashed line in FIG. 1 indicates the magneticfield measurement apparatus 90. The magnetic field measurement apparatus90 includes a first gas cell 10, a first measurement unit 11, a secondgas cell 20, a second measurement unit 21, coils 31 and 32, a controller33 and an output unit 40.

An object to be measured 50 is an object which is a measurement targetof a magnetic field, for example, a human body. Both the first gas cell10 and second gas cell 20 are cells in which gases such as alkali metalatoms are filled in internal space partitioned by a transparent memberof glass or the like.

As shown in FIG. 2, the first gas cell 10 is disposed in a positive zdirection which is determined in advance when seen from a position inwhich the object to be measured 50 is installed. The second gas cell 20is disposed in the positive z direction which is determined in advancewhen seen from the first gas cell 10. In other words, the object to bemeasured 50, the first gas cell 10 and the second gas cell 20 aresequentially arranged along the positive z direction. Accordingly, thefirst gas cell 10 is an example of a first gas cell which is disposed ina direction which is determined in advance when seen from a position inwhich the object to be measured is installed. In addition, the secondgas cell 20 is an example of a second gas cell which is disposed in adirection which is determined in advance when seen from the first gascell.

The first measurement unit 11 includes an irradiation unit whichrespectively irradiates pump light which has a circular polarizationcomponent and probe light which has a linear polarization componenttoward the first gas cell 10 and a detection unit which detects theprobe light which passes through the first gas cell 10. The irradiationunit irradiates the pump light and the probe light so as to cross overto each other or it is preferable to irradiate the pump light and theprobe light so as to be orthogonal to each other. The detection unitextracts the linear polarization component among the detected probelight, specifies an angle at which the polarization surface thereofrotates after passing through the first gas cell 10 and measures acomponent in the positive z direction of a magnetic field in the firstgas cell 10 based on the angle. Therefore, the first measurement unit 11is an example of a first measurement unit which irradiates light to thefirst gas cell and measures a component in a predetermined direction ofa magnetic field in the first gas cell.

The second measurement unit 21 includes the same configuration withrespect to the second gas cell 20 as the configuration of the firstmeasurement unit 11 with respect to the first gas cell 10 and measures acomponent in the positive z direction of a magnetic field of the secondgas cell 20 in the same manner as the first measurement unit 11.Therefore, the second measurement unit 21 is an example of a secondmeasurement unit which irradiates light to the second gas cell andmeasures a component in a predetermined direction of a magnetic field inthe second gas cell.

In the disposition shown in FIG. 2, pump light La1 is irradiated towardthe first gas cell 10 from the negative x direction when seen from thefirst gas cell 10 and pump light La2 is irradiated toward the second gascell 20 from the negative x direction when seen from the second gas cell20. Atoms such as alkali metal which are filled in the first gas cell 10and the second gas cell 20 are excited by these pump light beams.

In addition, probe light Lb1 is irradiated toward the first gas cell 10from the negative y direction when seen from the first gas cell 10 andprobe light Lb2 is irradiated toward the second gas cell 20 from thenegative y direction when seen from the second gas cell 20.Subsequently, a detection unit of the first measurement unit 11 detectsprobe Lc1 which passes through the first gas cell 10 and proceeds in thepositive y direction when seen from the first gas cell 10 and adetection unit of the second measurement unit 21 detects probe light Lc2which passes through the second gas cell 20 and proceeds in the positivey direction when seen from the second gas cell 20.

The coil 31 is disposed in the negative z direction when seen from theposition where the object to be measured 50 is installed and the coil 32is disposed in the positive z direction when seen from the second gascell 20. In other words, the coil 31 and coil 32 include the object tobe measured 50, the first gas cell 10 and the second gas cell 20 whichare interposed along the positive z direction.

In addition, the controller 33 is connected to the output of the secondmeasurement unit 21. The controller 33 is a section which generates andoutputs a signal in response to a measurement result by the secondmeasurement unit 21 and includes, for example, an amplifier whichamplifies the signal which is measured by the second measurement unit21, a filter unit which extracts the signal in a determined band and anadjustment circuit unit which adjusts a signal phase. The output of thecontroller 33 is connected to the coil 31 and coil 32. The coil 31 andthe coil 32 generate the magnetic field toward measurement space A inwhich the object to be measured 50, the first gas cell 10 and the secondgas cell 20 are disposed in response to the signal which is output bythe controller 33. To be more specific, a magnetic field of an inversephase to the signal in response to the measurement result by the secondmeasurement unit 21 is generated toward the measurement space A in whichthe second gas cell 20 is disposed so as to reduce the component whichis measured by the second measurement unit 21. By receiving the magneticfield of the inverse phase, the magnetic field which is received by thesecond gas cell 20 from the outside of the magnetic field measurementapparatus 90 is cancelled. Moreover, since the magnetic field of theinversed phase is received by the first gas cell 10, the magnetic fielddue to the difference in the disposition between the first gas cell 10and the second gas cell 20, that is, the magnetic field which is derivedfrom a fact that the first gas cell 10 is disposed near the object to bemeasured 50 compared to the second gas cell 20 remains in the first gascell 10. Therefore, the coil 31 and coil 32 are examples of magneticfield generation sections which include the object to be measured, thefirst gas cell, the second gas cell interposed along the directiondetermined in advance and generate the magnetic field toward the secondgas cell so as to reduce the component measured by the secondmeasurement unit.

The output unit 40 outputs the signal in response to the difference inthe component in the positive z direction of the magnetic field in thefirst gas cell 10 which is measured by the first measurement unit 11 andthe component in the positive z direction of the magnetic field in thesecond gas cell 20 which is measured by the second measurement unit 21.Therefore, the output unit 40 is an example of an output unit whichoutputs a signal in response to the difference in the components whichare respectively measured by the first measurement unit and the secondmeasurement unit. The magnetic field measurement apparatus 90 indicatesa value which shows the component in the positive z direction of themagnetic field in the object to be measured 50 on the basis of thesignal output by the output unit 40 to a user.

For example, in a case that magnetic field noise is received from theoutside which exceeds a range which is measurable by the firstmeasurement unit 11 or the second measurement unit 21, that is, adynamic range, unless there are the coils 31 and 32, which are providedin the magnetic field measurement apparatus 90, since either of thefirst measurement unit 11 or the second measurement unit 21 or boththereof become incapable of measurement, even though the output unit 40outputs the signal in response to the difference in the components whichare respectively measured by the first measurement unit 11 and thesecond measurement unit 21, it is difficult to indicate an accuratevalue which shows the magnetic field of the object to be measured 50 onthe basis of the signal thereof.

In the magnetic field measurement apparatus 90, even in a case in whichthe magnetic field noise which exceeds the dynamic range is received,since the magnetic field is generated toward the measurement space A bythe coil 31 and the coil 32 so as to reduce the component measured bythe second measurement unit 21, the magnetic field noise thereof isattenuated inside the measurement space A and it is possible to suppressthe influence from the magnetic field noise with respect to themeasurement of the magnetic field of the object to be measured 50.

In addition, the magnetic field which is generated by the coil 31 andcoil 32 toward the measurement space A is arranged to reduce thecomponent measured by the second measurement unit 21, however, since thesecond gas cell 20, which is a measurement target of the secondmeasurement unit 21, is disposed in a different position from the objectto be measured 50, there also is a possibility that the influence of themagnetic field noise received by the object to be measured 50 may not becompletely cancelled in the measurement space A. However, the first gascell 10 is disposed near the object to be measured 50 compared to thesecond gas cell 20 since other gas cells are not disposed between thefirst gas cell 10 and the object to be measured 50. Meanwhile, since thefirst gas cell 10 is disposed between the second gas cell 20 and theobject to be measured 50, the second gas cell 20 is disposed further outfrom the object to be measured 50 compared to the first gas cell 10. Asa result, since the magnitude of the magnetic field generated by theobject to be measured 50 is inversely proportional to the square of thedistance from the object to be measured 50, the second gas cell 20 hasdifficulty in detecting the magnetic field of the object to be measured50 compared to the first gas cell 10. Since the output unit 40 of themagnetic field measurement apparatus 90 outputs the signal in responseto the difference in the measurement results by the second measurementunit 21 of the second gas cell 20 and by the first measurement unit 11of the first gas cell 10, the influence from the magnetic field noisereceived by both the first gas cell 10 and the second gas cell 20 iscancelled and the influence from the magnetic field derived from theobject to be measured 50 is extracted. Therefore, even regarding thispoint, the influence from the magnetic field noise with respect to themeasurement of the magnetic field of the object to be measured 50 can besuppressed.

In addition, the magnetic field measurement apparatus 90 according tothe embodiment described above serves as a so-called first derivativegradiometer in which the output unit 40 outputs the signal in responseto the difference in the measurement results of the first gas cell 10and the second gas cell 20; however, the magnetic field measurementapparatus 90 may be configured by multiple stages in order to serve as asecond derivative gradiometer or a derivative gradiometer of higherorder.

Moreover, the magnetic field measurement apparatus 90 according to theembodiment described above is arranged to set only the component in onedirection, that is, in the positive z direction among the magneticfields as the measurement target; however, a magnetic field measurementsystem may be provided which measures a magnetic field of, for example,two dimensional space or three dimensional space by providing aplurality of the magnetic field measurement apparatuses 90 in eachindependent direction. The magnetic field measurement system is arrangedto set the components in a plurality of independent directions as themeasurement targets. Therefore, the magnetic field measurement system isan example of a magnetic field measurement system which includes aplurality of the magnetic field measurement apparatuses in eachindependent direction.

For example, FIG. 3 is a diagram which illustrates disposition of eachgas cell in a magnetic field measurement system provided with the twomagnetic field measurement apparatus 90. As shown in FIG. 3, also byproviding the configuration of the magnetic field measurement apparatus90 described above in the positive y direction, the component in thepositive y direction may be measured in addition to the component in thepositive z direction among the magnetic field of the object to bemeasured 50.

In this magnetic field measurement system, in addition to the first gascell 10 and the second gas cell 20 which are disposed in the positive zdirection when seen from the object to be measured 50, a third gas cell60 which is disposed in the positive y direction when seen from theobject to be measured 50 and a fourth gas cell 70 which is disposed inthe positive y direction when seen from the third gas cell 60 areincluded.

In the disposition shown in FIG. 3, pump light La6 is irradiated towardthe third gas cell 60 from the negative z direction when seen from thethird gas cell 60 and pump light La1 is irradiated toward the fourth gascell 70 form the negative z direction when seen from the fourth gas cell70. Atoms such as alkali metal which are filled in the third gas cell 60and the fourth gas cell 70 are excited by these pump light beams.

In addition, probe light Lb6 is irradiated toward the third gas cell 60from the negative x direction when seen from the third gas cell 60 andprobe light Lb7 is irradiated toward the fourth gas cell 70 from thenegative x direction when seen from the fourth gas cell 70.Subsequently, probe light Lc6 which passes through the third gas cell 60and proceeds in the positive x direction when seen from the third gascell 60 and probe light Lc7 which passes through the fourth gas cell 70and proceeds in the positive x direction when seen from the fourth gascell 70 are respectively detected and each of the magnetic fields of thethird gas cell 60 and the fourth gas cell 70 are measured by measuringrotational angles of the respective polarization surfaces.

The object to be measured 50, the third gas cell 60 and the fourth gascell 70 are interposed along the positive y direction by two coils whichare not shown. The two coils generate the magnetic field of the inversephase to a signal in response to the measurement result of the magneticfield in the fourth gas cell 70 toward the measurement space in whichthe object to be measured 50, the third gas cell 60 and the fourth gascell 70 are disposed. By receiving the magnetic field of the inversephase, the magnetic field received by the fourth gas cell 70 from theoutside of the magnetic field measurement system is cancelled. Moreover,since the third gas cell 60 receives the magnetic field of the inversephase, the magnetic field which is received by the third gas cell 60 andis not received by the fourth gas cell 70 remains in the third gas cell60.

As a result, from the prove light beams which pass through the first gascell 10 and the second gas cell 20, the component in the positive zdirection of the magnetic field in the object to be measured 50 ismeasured and from the probe light beams which pass through the third gascell 60 and the fourth gas cell 70, the component in the positive ydirection of the magnetic field in the object to be measured 50 ismeasured. Here, the irradiation directions of the pump light and theprobe light are simple example and aspects of the irradiation of theselight beams in the magnetic field measurement system is not limited tothe example.

The entire disclosure of Japanese Patent Application No. 2011-081807,filed Apr. 1, 2011 is expressly incorporated by reference herein.

What is claimed is:
 1. A magnetic field measurement apparatus,comprising: a first gas cell which is disposed in a predetermineddirection when seen from a position in which an object to be measured isinstalled; a second gas cell which is disposed in the direction whenseen from the first gas cell; a first measurement unit which irradiateslight to the first gas cell and measures a component in the direction ofa magnetic field in the first gas cell; a second measurement unit whichirradiates light to the second gas cell and measures a component in thedirection of a magnetic field in the second gas cell; a magnetic fieldgeneration section which includes the object to be measured, the firstgas cell and the second gas cell interposed along the above directionand generates a magnetic field toward the second gas cell so as toreduce the component measured by the second measurement unit; and anoutput unit which outputs a signal in response to a difference in thecomponents which are respectively measured by the first measurement unitand the second measurement unit.
 2. A magnetic field measurement system,comprising: a plurality of the magnetic field measurement apparatusesaccording to claim 1 in each independent direction.
 3. The magneticfield measurement system according to claim 2, wherein a plurality ofthe directions is orthogonal to one another.
 4. A magnetic fieldmeasurement method, comprising: irradiating light to a first gas cellwhich is disposed in a predetermined direction when seen from a positionin which an object to be measured is installed and measuring a componentin the direction of a magnetic field in the first gas cell by a firstmeasurement unit; irradiating light to a second gas cell which isdisposed in the direction when seen from the first gas cell andmeasuring a component in the direction of a magnetic field in the secondgas cell by a second measurement unit; generating the magnetic fieldtoward the second gas cell so as to reduce the component which ismeasured by the second measurement unit by a magnetic field generationsection which includes the object to be measured, the first gas cell andthe second gas cell interposed along the direction; and outputting asignal in response to a difference in the components which arerespectively measured by the first measurement unit and the secondmeasurement unit.